1, Field of the Invention
This invention relates to a method of improving the mechanical properties of solder and brazing alloys of the type which have coarse, acicular and/or dendritic intermetallic phases and to the product thereof. More particularly, the method of the invention involves subjecting a melt of the alloy, while in a semi-solid state, to vigorous shearing or vibration at the solid-liquid interface. Upon solidification, the intermetallic phase or phases precipitate as fine, non-dendritic structures.
2. Prior Art
Some recently developed solder and brazing alloys have a microstructure containing coarse, acicular and/or dendritic intermetallic phases of relatively high hardness, and melting points higher than those of the matrix alloys. These intermetallic phases may form bulky dendritic or pointed structures which extend throughout the length of the soldered or brazed joint and adversely affect the mechanical properties which are potentially obtainable. The intermetallic phases may also be interconnected and thus may form a brittle bridge between the joined parts which also degrades creep strength, adherence and other important properties.
For example, governmental restrictions on the use of lead-based solders in potable water supply means have led to the development of solder compositions which generally contain tin and/or indium. It is well known that most tin and indium-based alloys contain one or more intermetallic phases after solidification from the molten state.
In some operations the dendritic intermetallic phase is larger than the gap between the pieces which are to be soldered or brazed. When this occurs, on application of the solder or braze wire the molten alloy which fills the gap is deficient (commonly referred to as macrosegregation), i.e., the intended composition of the solder or braze is different from that which actually flows into the gap by reason of the fact that the intermetallic phase or phases do not enter the gap which forms the joint.
The prior art has used extrusion of these alloys as a means of breaking up these intermetallic phases. Extrusion has been successful in breaking up interconnected structures, but it does not change the acicular and/or dendritic shape thereof.
The prior art has also resorted to the expedient of adding small amounts of sodium to aluminum--silicon alloys in order to alter the microstructure, but this modification has not been successful in eliminating the dendritic structure of the intermetallic phase therein.
Definite problems therefore exist with solder and brazing alloys which contain coarse, acicular and/or dendritic intermetallic phases, which makes it impossible to realize the potential improvement in properties which a hard, high melting point intermetallic phase would otherwise confer if present in finely divided, non-acicular and non-dendritic form.